I personally have trouble reading the chat log during combat, not being used to combat in general (I'm more of a crafter and gatherer, where I have the time to actually think, not constant outright panic mode >_> ). Due to sensory issues, I also have a very hard time using spoken chat programs. Chat bubbles would be useful to me because they'd stand out more compared to the chat log.
My first MMORPG experience was Runescape. RS uses un-toggleable chat bubbles (without the bubble, just a line of text over the player's head). Originally RS didn't have a central way of selling things; everyone used trades and advertised their wares in popular banks around the world. Even now, you can still go to a certain world (RS characters aren't locked to servers, you can freely log into any one of them on the same character) and watch people trying to advertise their wares. If you go there, the chat log will zip past so fast that no one can read it, and the lines above people's heads can get quite cluttered (it can be hundreds of lines of text in a minute even without it being peak hours).
That sort of thing is really the sort of worst case scenario that I think a lot of people are worried about, lots of clutter without being able to turn it off.
Personally, I think there should be not one but two toggles for it if it ever is considered for implementation. First is instanced content, for which I think the bubbles should be defaulted to on. Secondly I'd suggest having open world chat bubbles (such as in camps and cities) defaulted to off. (Or have one toggle with several settings including all possible combinations of on and off for both instanced and non instanced situations, again defaulting to on in an instance and off outside.) That way, most players will have it on when it's important to have it on, but not be overwhelmed by it out in the popular camps/cities. Most people won't actually go through their settings to change anything unless it is something they think of as a hindrance, so setting it up from the start in such a way that you can still use it to communicate in dungeons from the start without it being a problem elsewhere would be the way to go.
However, I don't think it's necessary to get up in arms about the issue. It would definitely be handy for some people (and there's no way anyone can make a serious statement about "most" players from reading a forum, considering only a very small percentage of people actually read - let alone participate in - a forum and they tend to only be a very limited range of types, with many more prefering to just play the game). Handy and quality of life is good but not crucial. If it doesn't get implemented (which is looking to be the case, at least for the currently foreseeable future), then nothing major will be lost. It will be annoying when people yell at me after a fight for not seeing something they said in the chat, or for being slow to do something because I was checking the chat, but I'll live and so will they. Most people are able to use spoken chat programs and I really think that is the best way to go for real time communication during combat if you can use them (much more effective than chat bubbles which won't even serve their purpose if your camera isn't facing that player and which still require typing which will get in the way in combat situations).
So in the end, I think chat bubbles on in dungeons would be very nice but I also don't mind the devs saying no. *Shrug*
One thing I definitely do NOT support however is the demanding nature of many of the people in this thread. You paid/pay for a product, yes, but that does not automatically give you the right to demand that the game be developed in any one way or another. You got what you paid for (basically the main storyline and basic dungeons, crafting and gathering, followed by updates on a semi regular basis as SE decides), a game that was designed by Yoshi&co, and if they make design decisions that you personally don't agree with then so be it. They will not and cannot please everyone and no one is asking them to. They will make the game in their own vision. Sometimes they will see something that the players want and decide that they want it too, but do not confuse that with players being allowed to design the game in their own image. If 50 people "liked" a thread saying that the sky should be gold with purple polka dots, by your reasoning Yoshi&co should implement that because that's what "most" players want. It's ridiculous. No. Let the people who make the game make the game. Be happy when they choose to implement something that you want too, but that's a nice surprise, not something to be demanded. Sheesh...